1. Field of the Invention
This invention relates to an active jaw for a power tong, a power tong including such an active jaw and a method of changing the capacity of such a power tong.
2. Description of Related Art
During the construction of an oil or similar well it is necessary to connect successive lengths of pipe. This is achieved by a threaded connection.
Conventionally, a tool known as a power tong is used for gripping one length of pipe and rotating it relative to the next to make or break the joint.
Known power tongs comprise a body, a rotary rotatably mounted in said body and at least one active jaw which, on rotation of the rotary is cammed against a pipe in the rotary and grips it for rotation with the rotary.
In known arrangements the camming action is generated by a cam member which is bolted to the rotary and is shaped so that the active jaw will be cammed against the pipe on rotation of the rotary relative to the active jaw in one sense and will be released on rotation of the rotary relative to the active jaw in the opposite sense.
In order to reverse the screwing direction of one known power tong with only one active jaw, the active jaw must be removed and remounted on the opposite side of the rotary. On rotation of the rotary relative to the active jaw in the opposite sense the active jaw is cammed against the pipe by a second cam member. This is a time-consuming process. Furthermore, the power tong must be removed from the pipe to enable the active jaw to be moved.
In another known power tong two cam members are bolted to the rotary. When the rotary is rotated relative to the active jaw in either direction from a neutral position the active jaw is applied to the pipe.
Another problem with known arrangements is that they can only be used in relation to a very limited range of pipe sizes. Outside of this range the jaws do not grip the pipe properly and, as a result, the surface of the pipe becomes damaged.
One solution to this problem has been to use different cam members for gripping pipes of different sizes. However, removing and replacing cam members is a time-consuming exercise. Furthermore, the range of pipe sizes which can be gripped satisfactorily cannot be substantially increased by this method.